a-lytic protease is a 198 amino acid bacterial serine protease that is secreted from Lysobacter enzymogenes. Before secretion it exists as a 40 kD precursor, consisting of a signal sequence, a PRO region, and the mature domain that eventually becomes the active protease. A number of studies have shown that the PRO region is required both in vitro and in vivo for proper folding of a-lytic protease. In vivo the protease expressed without the PRO region is not secreted. In vitro the denatured protease will not refold without the addition of PRO region. The PRO region is also a sub-nanomolar inhibitor of the native enzyme. It is clear that the interactions between the PRO region and the mature protease are extensive and may shed some light on how proteins fold and why a-lytic cannot fold by itself. We will be studying crystals of the PRO region.